The purpose of the proposed study is to investigate the nature of visual spatial neglect in Parkinson's disease (PD). Non-motor symptoms of PD, such as neglect, have recently been acknowledged as an important area of investigation, given that cognitive and perceptual deficits have a large impact on quality of life in patients. PD patients with disease onset on the left side of the body (LPD) show spatial hemineglect in response to stimuli presented to the left visual hemifield (LVF), whereas patients with disease onset on the right body side (RPD) perform similarly to healthy adults. Hemineglect in LPD is manifested as patients estimating the center of a line to be to the right of center, viewing objects as smaller if they appear in the LVH than if they appear in RVF, and biased initial visual exploration to the right hemifield. The functional and neural mechanisms underlying hemineglect in PD are unknown. One possibility is that space is perceptually compressed in the LVF for LPD, and another is that faulty endogenous or exogenous attentional processing results in a distortion or weakening of visual signals in the LVF. The proposed study will test these competing hypotheses by using psychophysical assessment of perceptual and ocular-motor responses in left and right visual hemifields in LPD, RPD, and age matched control participants. Based on the role of the brain structures that are pathological in PD, such as parietal lobes and areas basal ganglia circuits modulating superior colliculus activity, it is hypothesized that attention, rather than spatial compression, accounts for the hemineglect results. PUBLIC HEALTH RELEVANCE: Visual and cognitive impairments such as spatial neglect are among the non-motor manifestations of PD that adversely impact the quality of life of patients and their caregivers, but are often overlooked by clinicians. Understanding the nature of spatial neglect in PD will provide a basis for targeted treatment strategies to compensate for navigational difficulties.